Add a shown_on_calendar field to Course model

[alexdlaird]

This would also allow whether or not a class should be shown on the calendar or not to be a default preference that is remembered from session to session (it's currently through a cookie, which can be a little finicky for the user experience).

• Add "shown_on_calendar" field to "Course" model
• Consider removing "shown_on_calendar" from "CourseGroup"
• When a class is selected/deselected from the "Classes" dropdown, PATCH the "Course" to enable/disable the "shown_on_calendar" field, which should be respected on page load

[alexdlaird]

Closing, as the Helium project is not currently active.